It was achieved by keeping track of existence of effective_comment_vote_operation inside current blob of blocks for each vote record and splitting update into two separate queries - one that updates and skips overwrites of rshares and weight (when only ineffective votes were present meaning default rshares/weight) and the other that updates existing records (when new data came from effective_comment_vote_operation). Unfortunately it is not clear if it would even be possible to do it in single query (but two queries are not slower than original version in this case).